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The end of Kepler? That would be universally bad

As I write this, engineers at NASA and Ball Aerospace & Technologies have their fingers crossed they’ll be able to restart the stricken Kepler space observatory, which has been in hibernation mode…

Two of Kepler’s four reaction wheels have stopped working, which may mean the end for the spacecraft. NASA/Kepler mission/Wendy Stenzel

As I write this, engineers at NASA and Ball Aerospace & Technologies have their fingers crossed they’ll be able to restart the stricken Kepler space observatory, which has been in hibernation mode since May 15. If they can’t, the telescope’s haul of planet candidates – 3,277 at the last count and including many Earth-like systems – might have come to an end.

Kepler has been idling roughly 64 million kilometres from Earth since the failure of one of its reaction wheel 4 – one of four mechanisms used to orient the spacecraft and stabilise its gaze. The plan, by no means assured of success, is to try to re-engage either the stalled-out wheel 4, or another that failed shortly after launch in 2009.

If this doesn’t work, the loss will be huge.

A once-in-a-lifetime mission

The section of Milky Way that includes the Kepler field of view. NASA

Kepler has been the most revolutionary facility the science of astronomy has seen in more than two decades. It’s hard to get across to other astronomers - let alone the general reader - just how exquisitely Kepler measures the amount of light coming from its target stars as it searches for the tiny repeating dips in that light that reveal the presence of an otherwise unseen planet.

Most astronomers who study the distant universe tend to think they’ve done pretty well if they measure the brightness of an object to a precision of 10%. Those who study stars in our galaxy (or in nearby galaxies) think they’ve done pretty well if they measure a brightness to a precision of 1%. And anyone who routinely measures a brightness to 0.1% can justifiably think of themselves as a “god” of the science of photometry.

But Kepler leaves the rest of us in the dust. It measures its target stars at precisions of ten parts-per-million or better. That’s 0.001%, or 100 times better than the mere “gods” of ground-based photometry can do.

And Kepler must reach those levels of precision if it’s to achieve its primary mission goal - detecting Earth-like planets orbiting sun-like stars.

Earth-like planets, you see, are very, very small. And stars are very, very big. So the amount of star light blocked by an Earth-sized planet is just a tiny (and not entirely coincidental) 10 parts per million, which has allowed Kepler to give us our first, albeit indirect, glimpses of other “Earths”.

Kepler 22’s planetary system compared to the Earth. NASA/Kepler mission

Keeping watch

One of Kepler’s key strategies for success has been to carry out its observations in as “differential” a fashion as possible - that is, the satellite looks at a set of stars hour-after-hour, day-after-day, week-after-week, for periods of up to three months.

In that time it tries to keep absolutely everything about the satellite the same, so the only thing that varies is the brightness of the stars.

And that requires keeping the telescope pointed at the same patch of the sky to a very high level of precision. This is done with the aid of its “reaction wheels”, two of which are shown in the picture below.

Schematic of Kepler showing two of the four reaction wheels used to control the craft. NASA

If you or I want to turn around, we can plant our foot on the ground, which gives us something to push against. In space, Kepler has nothing to push against, so it relies on a physical effect called the “conservation of angular momentum”.

By having a small wheel spin rapidly in one direction, the balancing nature of angular momentum will slowly spin the rest of the satellite in the opposite direction. By using three of these wheels in combination, it can turn itself in any direction or, more critically, make small adjustments to stay accurately pointed at the same part of the sky without moving.

But the trick is, it needs three functional wheels. Two just doesn’t cut it - and sadly two is what it has right now.

One last chance

NASA (having some experience with hurling widgets into space) wisely designed Kepler with an additional, redundant reaction wheel - wise because, soon after launch in 2009, one of those four wheels failed.

The Kepler mission was originally designed to be five years long, but has been so successful it was extended last year to a total mission of seven years. So for the past four years everyone concerned has held their breath, hoping nothing untoward happened to the other wheels for at least another three years.

Sadly, as we now know, a second wheel failed in May. As a result, NASA placed the satellite into what’s known as Point Rest State – a state that minimises fuel usage. Then a lot of engineers sat down to have a really good think about what to do next.

John Troeltzsch of Ball Aerospace & Technologies was quoted this week as saying:

We are just going to try to move the wheels in the opposite direction from where they were going when they stopped working. If we can get it moving freely, we can spin it for a while and let it coast to a stop. Then we can understand the level of friction.

Having the wheels operate in the way they did before the stall was unlikely, Troeltzsch added, before pointing out the obvious, that:

We can’t send a repair man out.

Astronomers acknowledge successfully restarting the wheels will be a long shot, but there really aren’t any other options.

Kepler’s current four-year mission is the absolute minimum needed to detect small planets in roughly one-year-long orbits. Extending that mission to seven years is critical to shrinking the large range of potential planet frequencies allowed by the current data, and nailing down just how common Earth-like planets really are.

So please – like the engineers – do all keep your fingers crossed.

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4 Comments sorted by

  1. Greg North

    Retired Engineer

    So how long did Kepler take to get out to where it is Chris and is there any possibility that in lieu of a repair fella or gal that can despatch some sort of a robot that could attach itself to Kepler with a couple of auxilliary wheels.
    I imagine the smart guys at NASA and Bell may have even contemplated that but then again focus can often mean being too focussed.

    1. Chris Tinney

      Professor and Associate Dean (Research) at UNSW Australia

      In reply to Greg North

      Kepler lies in an "Earth-trialing" orbit. That it is actually orbits the Sun, not the Earth, and has been lagging behind the Earth over the last 4 years (see .e.g.

      Unlike Hubble which sits in a low-Earth orbit and was designed with regular servicing missions by the Shuttle in mind, Kepler was not designed with any 'replaceable parts' ... which means it would probably cost just as much to (a) reach it and (b) repair it, as to build an entirely new one and launch it again.

    2. Chris Harper


      In reply to Chris Tinney


      From what you haven't said I assume Kepler needs to be this far from any sizeable body in order to function, minimizing gravitational perturbation perhaps. I also assume that it lacks either the fuel or motors, or both, to return it to Earth orbit for repair.

      Would adding a return facility to a Kepler Mk II render it unfeasibly expensive?

    3. Chris Tinney

      Professor and Associate Dean (Research) at UNSW Australia

      In reply to Chris Harper

      Chris - the issue isn't gravitational perturbation - its the fact that orbiting close to the Earth means there are regular times when your target is invisible because of the Earth getting in the way, as well as regular instabilities associated with moving into (and out of) the Earth's magnetic field. For this mission, being a long way frm the Earth is the right place to be. And yes, a return capability would be almost certainly be unfeasible expensive. And it would take another 4 years to come back as well (that's how long it has taken to get to its current position).